2.1. Introduction
For over ninety years the aerospace industry has relied on primers and pretreatments containing hexavalent chromium (Cr(VI)) as the most effective corrosion inhibitor for aluminum and aluminum alloys. Unfortunately, Cr(VI) is a known carcinogen and exposure can have serious health consequences including lung cancer. The U.S. Occupational Safety and Health Administration (OSHA) recommends if possible, substitution with paints and coatings that do not contain Cr(VI) (“Controlling Exposure to Hexavalent Chromium in Aerospace and Air Transport Painting” OSHA Fact Sheet FS-3650, March 2013). Accordingly, there is a long-felt need for corrosion resistant paints and coatings that do not contain Cr(VI) in both painting and maintenance. Effective alternatives to Cr(VI) for corrosion inhibition would reduce chromium use from the initial manufacturing processes, over the maintenance cycle, and to ultimate disposal processes.
2.2. Electropolymerized Conducting Polymer Coatings
Electropolymerization of aniline to produce PANI is a well-known process. Pournaghi-Azar and Habibi disclose PANI coatings on aluminum and aluminum-platinum surfaces from sulfuric acid solutions (Pournaghi-Azar and Habibi Electrochimica Acta 52 (2007) 4222-4230). PANI has been presented as a corrosion protection system for active metal surfaces through a passivation mechanism. For example, Biallozor and Kupniewska review electrodeposition of conducting polymers on active metals including use of PANI on aluminum for corrosion protection (Biallozor and Kupniewska Synthetic Metals 155 (2005) 443-449).
On aluminum alloys, PANI has been shown to function as a barrier to corrosion in its basic or de-doped form. In its doped form utilizing traditional sulfonic acid dopants, corrosion protection has not been observed due to the continuous oxidation of the metal by the film resulting in delamination. Specifically, Conroy and Breslin disclose PANI coatings prepared on pure aluminum from a tosylic acid (p-toluenesulfonic acid, CH3C6H4SO3H) solution (Conroy and Breslin Electrochimica Acta 48 (2003) 721-732). They observe the formation of aluminum oxides, however they note that “there was little evidence of any corrosion protection by the polyaniline coatings.”
U.S. Pat. No. 6,328,874 (Kinlen et al.) discloses electrodeposition of PANI on aluminum and aluminum alloys with polystyrene sulfonic acid (PSSA) and sulfuric acid PSSA mixtures and the characterization of aluminum oxide coatings formed by the process.
2.3. Solution-Based Conducting Polymer Coatings
Others have reported solution deposited PANI films; PANI graft polymers with poly(ethylene oxide)(PEO); and PANI poly(ethyleneoxy)-3-aminobenzoates (PEAB) copolymer grafts with PEO; and PANI doped with 2,5-dimercapto-1,3,4-thiadiazole (DMcT) for preparation of capacitors and solid-state rechargeable batteries (Moon and Park Solid State Ionics 120 (1999) 1-12).
U.S. Pat. No. 6,942,899 (Kendig et al.) discloses solution deposited PANI films on aluminum or aluminum alloys doped with organic acids such as DMcT or 2-mercapto thiazoline.